Gas separation system

ABSTRACT

In a gas separation system utilizing reversing heat exchangers which employ a reheat stream to facilitate heat exchanger temperature control and cleaning, the improvement including at least a pair of particulate filters placed at the heat exchanger feed stream outlets arranged to automatically be cleaned by a portion of the reheat stream without producing objectionable pressure differentials within the gas separation system.

REVERSING HEAT EXCHANGERS OXYGEN ll AIR INTAKE AIR COMPRESSOR LIQUEFIEREXPANDER United States Patent 1 1 1111 3,739,593 Schauls June 19, 1973[54] GAS SEPARATION SYSTEM 2,835,115 5/1958 Karwat 62/18 1 2,924,078 21960 T d 62 14 [75] Inventor: JamesJ. Schauls, La Crosse,W1s. Sum a l[73] Assignee: The Trane Company, La Crosse, Primary Examiner-NormanYudkoff I Wi Assistant Examiner-Arthur F. Purcell [22] Filed: Dec. 41968 Attorney-Lee E. Johnson, Arthur 0. Andersen and Carl M. Lew1s [21]Appl. No.: 781,201

, [57] ABSTRACT [52] U.S. Cl 62/14, 62/29, 62/38 In a gas separationsystem utilizing reversing heat eit- [51] Int. Ch j 3/ j j 5/ changerswhich employ a reheat stream to facilitate [58] Field Of Search 62/12,13, 14, 15, heat exchanger temperature control and cleaning, the

H 3 improvement including at least a pair of particulate filters placedat the heat exchanger feed stream outlets References Cited arranged toautomatically be cleaned by a portion of UNITED STATES PATENTS thereheat stream without producing objectionable 2 460 859 1/1949 Trumpler0 62,14 pressure differentials within the gas separation system.2,863,296 10/1958 Newton 62/14 6 Claims, 2 Drawing Figures 2,584,3812/1952 Dodge 62/14 I 2,753,701 7/1956 Palmer 62/29 FRACTI DNATOR FILTER25 FILTER COOLER GAS SEPARATION SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to improvements in gas separation systems particularlyof the type very aptly disclosed in US. Pat. No. 2,460,859, from whichmuch of the instant disclosure has been taken for purposes of settingforth the environment of this instant invention.

This invention relates to an improved method and apparatus for theseparation of gas mixtures containing lower and higher boilingcomponents and other components which boil at still higher temperatures.More specifically, it is concerned with a continuous method for theseparation of air into a substantially oxygencontaining fraction, and asubstantially nitrogencontaining fraction and elimination from the airof undesirable impurities, such as water vapor and carbon dioxide orother high boiling components.

The separation of low-boiling gas mixtures, for example air, intorelatively pure components heretofore has been accomplished by processesinvolving expansion, liquefaction, and fractionation. In processes ofthis character, heat exchangers have been employed to precool the gasmixtures by counter-current heat inter change with backward-returningcold product material. When the process is conducted under highpressure, the undesirable higher boiling impurities, such as water vaporand carbon dioxide or hydrocarbons, are eliminated by the employment ofswitching heat exchangers or by absorbers and adsorbers. However, whenthe separations are conducted under relatively low pressures, only theaforementioned switching heat exchangers generally are utilized toremove the higher boiling components. One major difficulty prevalent insuch use of switching heat exchangers arises from the fact that thewater, carbon dioxide, or other comparatively higher boiling componentsin the air will precipitate therefrom as a solid and accumulate in theexchangers in deposits great enough to plug up the passageways of theseves' sels. The plugged exchanger then has to be switched with anunplugged one and thawed out before again being used. Consequently, theeconomy of a system using switching exchangers in this manner is greatlyreduced because of the necessity to cover the cold losses.

The separation of air, or other normally gaseous mixtures, into therelatively pure components also has been accomplished by a method whichincludes compressing and precooling of the mixture, liquefaction at theinitial pressure of a portion of the mixture by heat interchange withcold products of the separation, the expansion of another portion withexternal work, fractionation of the two portions in a commonfractionating tower at at the lowerpressure, and backward return of theproducts of separation.

A counter-current reversing cold exchanger system have been developedwhich permits a simultaneous and efficient heat interchange betweenpassageways containing counter-currently flowing streams of air andbackward returning cold products. This exchanger comprises a pluralityof parallel paths for the fluid in each passageway which are so metalbonded together as to establish a metal to metal thermal contactthroughout the whole contact length of the vessel. Likewise the severalpassageways of the exchanger are joined with metal to metal contact.

These reversing heat exchangers are also normally utilized to removealmost all of the higher boiling impurities from air, or other gaseousmixtures, particularly for separations conducted at relatively lowpressures, such removal being accomplished by periodically alternatingthe flow of warm incoming feed and a backwardreturning cold productbetween at least two passageways of the exchanger. That is, during onehalf of the reversing cycle when the air is being cooled, water andcarbon dioxide, for example, are precipitated therefrom and accumulatedin solid or liquid phase on the metal surfaces of the passageway throughwhich the air at that time is flowing. Then, before the accumulation hasbecome great enough to plug that passageway the counter-current heatexchange with each other to an extent sufficient to establish conditionssuitable to bring about re-evaporation of the deposited solid impuritiesby less mass quantity of expanded output product than the air from whichthey are precipitated.

, Despite the effectiveness of this system in removing carbon dioxideand other impurities by deposition upon the metal heat exchangersurfaces, it has been found that minute quantities of such impuritiesare carried from the heat exchangers by the feed stream down through theprocess steps. Because these steps involve lower temperatures, theseimpurities remain in the solid state and may accummulate to foul theoperation of valves, nozzles, instrument lines, etc. While cycloneseparators have been used at the outlets of the heat exchangers toentrap such impurities, they are ineffective in removing particulateimpurities in the order of 2 to 10 microns. The use of porous filters atthis point in the system would ordinarily be considered impracticalbecause of the substantially large pressure drops they would impart tothe waste stream.

SUMMARY OF THE INVENTION The instant invention pertains to the methodand apparatus for using a particulate filter in a gas separation systemsuch as of the type above described wherein such filter is arranged toremove particulate impurities from the feed stream leaving the heatexchangers without imparting an objectionable pressure differentialwithin the system upon reversing of the heat exchanger streams.

It is another object of this invention to employ a particulate filter insaid gas separation system for the purpose of entrapping particulateimpurities in the order of 2 to 10 microns.

More particularly it is an object of this invention to employ aparticulate filter in said gas separation system for the purpose ofentrapping carbon dioxide particles in the order of 2 to 10 microns.

It is a further object of this invention to provide a particulate filtermeans to said gas separation system for removing impure solidifiedparticles which may be cleaned without imparting undesirable pressuredifferentia ls within the gas separation system.

It is still another object of this invention to provide a particulatefilter means to said gas separation system which is automaticallycleaned upon reversing of the feed and waste streams.

Other objects and advantages may become apparent as this descriptionproceeds to describe the invention.

In the accompanying drawings, FIG. 1 is a diagrammatic flow sheetdepicting one exemplary embodiment of my invention in connection with anillustrative processing arrangement for producing pure oxygen from airby a continuous low pressure method, involving re versing heatexchangers, which is capable of operations of long duration. While theitems of equipment shown in the drawing for illustrative purposes areparticularly designed for'use in a small mobile plant adaptable toinstallation on the chassis of a motor truck, it is to be understoodthat the present invention is equally adaptable to large commercialplant installations. Furthermore, the present invention is not limitedin its scope to processes involving the separation of air, since it isequally applicable to separations of other gaseous mixtures whichcontain higher boiling impurities such as,

, for example, rare gases and normally gaseous hydrocarbons. In theembodiment shown in FIG. 1, backwardlyreturning low pressurenitrogen-containing product utilized in the auxiliary heat interchangeris used to carry out the cleaning of the filter.

FIG. 2 shows a portion of the flowsheet of FIG. 1 and illustrates analternative form of the invention in which a portion of the compressedincoming air is used to carry out the cleaning of the filter.

Referring now to the drawing, atmospheric air which in this instance isat 120 F. and atmospheric pressure which preferably has been prefilteredis drawn into the first stage compression chambers l, 2, 3, and 4 ofcompressor 5 through intake ports 6, 7, 8, and 9. Compressor 5 is shownon the drawing as being a comparatively small and compact air cooledcompressor which is driven by an air cooled internal combustion engine10, especially adaptable for use in a mobile type plant. The partiallycompressed air is discharged from the first stage compression chambersthrough outlet lines 11, 12, 13, and 14 respectively into line 15 andconveyed therethrough to intercooler 16. In intercooler 16, thetemperature of the air is reduced to about 135 F. Usually, as there isnot condensation of water from this temperature reduction in intercooler16, the cooled air is passed directly to compression chambers 17 and 18by way of lines 19 and 20 respectively for a second stage compression tothe desiredfinal operating pressure which, in the present instance,-isabout 105 lbs. per square inch absolute. At this pressure the air leavescompression chambers 17 and 18 through lines21 and 22 respectively, atabout 410 F., whereafter they are combined in line 23 for passage toaftercooler 24. Aftercooler 24 and intercooler 16 comprise finned-tubeheat exchangers cooled by a blast of fan-driven air. In passing throughthe aftercooler the temperature of the air again is reduced to about 135F. This reduction in temperature at the final pressure condenses most ofthe water vapor which is withdrawn by a means not shown on the drawing.The partially dried air is then passed to filter chamber 25 by way ofline 26 for removal of oil carried over from the compressor, or otherimpurities such as light hydrocarbons. Upon leaving filter 25 throughline 27 the air may have the residual amounts of water removed bypassage through an absorber means, again'not shown on the drawing. Inany event, the air is discharged from line 27 into the inlet port ofreversing valve 28. Exchanger 29 is shown in line 27 for heating thefiltered air with hot effuent from the second stage of compressor 5which is taken from and returned to line 23 by lines 30 and 31respectively. Since heat exchanger 29 is an auxiliary apparatus fordelivering an emergency supply of hot air through line 27, it normallyis not in use.

During the operation in one phase of the reversing cycle, the nowsubstantially dried and filtered air is taken through the four-wayreversing diverter valve 28 and passed through the inner annuli 32, 33,34, and 35 of revering heat exchangers 36, 37, 38, and 39 respectively.These reversing exchangers may have their annuli and center passagewaypacked with a continuous coil of edgewound metal ribbon 87 closelybonded to the metal walls of the exchangers, or the exchangers may beotherwise constructed of the fiat plate type, a primary requisite beingthat the passages are metal to metal bonded to provide a small thermalresistance for the conduction of heat. Optionally there may be only oneexchanger having the necessary heat exchange surface. In passing throughannuli 32, 33, 34, and 35, the pressured air is in counter-current heatinterchange with the cold products of its subsequent separation, asshall hereinafter be described. By such heat interchange the temperatureof the pressured air is lowered during its passage through heatexchangers 36, 37, and 38 to approximately -l50 F., and it is at thistemperature that the air enters the last heat exchanger 39 in theseries. In heat exchanger 39 the temperature of the air is furtherreduced to the order of about 253 F. after which it is withdrawn throughline 40, filter 88,'unidirectional float check valve 41 and line 42 andintroduced into surge drum 43. During its course through the foregoingtemperature reduction, the pressured air deposits residual amounts ofwater on the metal surfaces of the exchangers as'liquid and frost andsimilarly its carbon dioxide constituent is laid down as a solidnormally in the last annulus 35 of exchanger 39. Any carbon dioxide snowwhich may have been carried out of exchanger 39 is trapped in filter 88.Filter 88 may employ any of a number of dry filter media capable offiltering particles in the order of 2 to 10 microns at about 250 F.These would include: Type -304 stainless steel wire mesh as used in anumber of pleated mesh filters sold by Fluid Dynamics, Inc., New York,N.Y.; DYNEL, a modacrylic staple fiber of vinyl and acrylic originmanufactured by E. I. Du Pont de Nemours and Co., Inc., Wilmington,Delaware; Media No. FM-004 and FM-003' fiberglass manufactured by OwensCorning Fiberglass Corp., Toledo, OH; and sintered metal filters made byPacific Sintered Metal Co. of Gardena, Calif.

The purified and cooled air leaving surge drum 43 through line 44 isdivided into two fractions. The larger fraction which represents in thepresent instance approximately 59 percent of the pressured air is takenthrough line .45 to liquefier 46 for heat interchange with coldbackward-returning, nitrogen-containing product. By passing through coil47 of the liquefier the temperature of this portion of the pressured airis again further reduced to about 274 F. to effect partial condensationand in this condition the air is thereafter introduced by way of line 49into the inside of reboiler calandria 48 of fractionator 50. Since thetemperature of the partially liquefied air is of the order of about 274F., it is warmer than the bath of liquid oxygen in which calandria 48 issubmerged. Consequently, the air gives up heat to reboil the bottoms offractionator 50 and in so doing reaches a temperature of about 278 F.This temperature causes total condensation of the air to liquid whichliquid is thereafter removed from calandria 48 through line 51 forpassage through carbon dioxide filter 52 and expansion through valve'53into the top of fractionator 50. Simultaneously, the smaller portion ofthe air from line 44, representing approximately 41 percent thereof, isintroduced by way of line 54into expander 55 wherein its pressure isreduced with work to about 25 pounds per square inch absolute and itstemperature correspondingly lowered to the order of --304 F. Under theseconditions the expanded 'air is caused to flow through line 56 havingsurge drum 57 disposed therein, and injected as vapor feed intofractionator 50 at an intermediate point somewhat below the point ofintroduction of liquid air from line 51. Bypass line 85, having valve86, connects line 56 with line 71 for starting up purposes during theperiod when the air is incompletely cooled.

Vapor to liquid contact is secured in fractionator 50 which brings aboutseparation of the air into a bottoms product which is essentially pureoxygen and an overhead product which contains a preponderance ofnitrogen. Pure oxygen vapors are removed from fractionator 50 throughdrawoff line 58 located immediately above the liquid body of thismaterial in the reboiler section of fractionator 50. Any entrainedliquid oxygen is sepathen effected by means of an additional annulus. inany rated from the vapors in separator 59 and returned to thefractionator through line 60. Vaporous oxygen leaves the top ofseparator 59 through line 61 at a temperature of about 288 F. and ispassed through the center passages 62, 63, 64, and 65 respectively ofreversing exchangers 39, 38, 37, and 36 for countercurrent heat exchangeagainst incoming pressured air which is now passing through annuli 32,33, 34, and 35 in the present phase of the reversing cycle before it isdischarged at a final temperature of 126 F. as product through line 66.I

The nitrogen-containing output product is removed as vapor from the topof fractionator 50 through line 67 at a temperature which is of theorder of about -290 F. Usually, all of this product is passed from line67 into line 68 for the counter-current heat interchange in liquefier 46in which event it is returned to line 67 through line 69. However, itmay be desirable at times to bypass a proportion of thisnitrogen-containing stream around the liquefier in which case valve 70is used to control the bypassed proportion. In any event,

the total nitrogen-containing stream is caused to flow from line 67 toline 71 at a temperature of about 275 F. for backward return through theaforementioned reversing exchangers 39, 38, 37, and 36 respectively. Atemperature of about 275 F. at the inlet to the cold end of theexchanger 39, however, is too cold, relative to an exiting temperatureof --25 3 F. of the pressured air at this end to evaporate all the solidcarbon dioxide in the reversed passage 80 of the exchanger through whichthis output product now flows in the present phase of the reversingcycle. Therefore, in accordance with the present invention, a definiteproportion of this cold product is diverted from line 71 through line 72to heat interchange means 74 for abstracting heat in a desired mannerover desired areas of exchanger surface in reversing exchanger 39. Theflow through exchanger means 74 is controlled in an amount which isdependent upon the operating conditions in the system by throttlingvalve 73 in the outlet line 75 for exchanger means 74. Although the heatinterchange means 74 is shown as a coil circumferentially aroundexchanger 39, the invention is not necessarily limited to this form ofheat exchange means. For instance, the cold product flowing from line 72may be passed as readily through any passageway exchanger 39 such as,for example, the center passageway 62 and the oxygen cold interchangeevent the diverted proportion of cold product leaves heat interchangemeans 74 at a somewhat higher temperature level through line 75,whereafter it is returned to the main stream of cold product emittingfrom line 71 through control valve 76 and the commingled material, inthe present phase of the reversing cycle, is then caused to pass throughline 77, unidirectional flow check valve 78 and line 79 into outerannulus 80 of exchanger 39.

In the instant invention, a small portion of the cold products leavingheat exchange means 74 is directed from line 75 at a point upstream ofvalve 73 through line 89, throttling control valve 90., line 91, tounidirectional flow check valve 92 and backwardly through filter 93 tojoin with line 79. The passage of a portion of the cold productsbackwardly through filter 93 will cause particulate carbon dioxide andother impurities entrapped therein on the preceeding reversing cycle ofoperation to subline and be removed from the filter 93. Because theportion of the gas passing backwardly through filter 93 as controlled byvalve 90 is very small compared to the total amount of gas being passedbackwardly through passage 80 of heat exchanger 39, there is littledisruption of the temperature and pressure balance established withinthe heat exchanger 39. In this manner the temperature of the commingledvapors as they enter annulus 80, in the present instance, is made to beof the order of about 262 F. which establishes a temperature differenceat the cold end of exchanger 39 between the incoming pressured air andbackwardreturning product of about 9 F. The 9 F. difference is less thanthe maximum allowable temperature difference between the air and productnecessary to provide for vapor pressures of the carbon dioxide in thesegases that will satisfy the vapor pressure and volume ratio relationshipat which complete evaporation of the solid carbon dioxide, orcarried-over ice, deposited in annuhis 80 during its previous use forthe cooling of the pressured air, can be accomplished. The "nitrogen gaswith evaporated impurities then passes through annuli 81, 82, and 83 tovalve 28 where it is discharged at 84.

l have now described the operation of my gas separation system operatingunder one cycle of operation during which solidified carbon dioxide andother particulate impurities are removed from filter 93 and annulus 80of heat exchanger 39. During this cycle of operation solidified carbondioxide and othersirnilar particulate impurities are deposited on thewalls of annulus 35 of heat exchanger 39 as well as entrapped withinfilter 88. After sufficient accumulation of these deposits valve 28 isreversed sending the pressurized air through annuli 83, 82, 81, and 80of heat exchangers 36, 37, 38, and 39 respectively. This cooled air is.then passed through filter 93, check valve 41' into line 42. Thenitrogen cold product passing in line 77 passes through check valve 78'through line 40 to annuli 35, 34, 33, and 32 of heat exchangers 39, 38,37, and 36 respectively. A

An alternative embodiment of the invention involves utilization of aportion of the compressed incoming air, after passage through theexchanger, as the cooling medium for use in the auxiliary heatinterchanger. This embodiment is illustrated in FIG. 2 in which partssimilar in function with similar parts in FIG. 1 are identified by thesame reference numeral as in FIG. 1, with the subscript a. In FIG. 2 aportion of the compressed incoming air, after passing through exchanger39a, is diverted from line 42a into line 72a which connects withauxiliary heat interchange means 74a. Line 75a connects the exit of heatinterchange means 74a with line 42a down stream of valve 76a located inline 42a. By thus connecting lines 72a and 75a to line 42a andrelocating valve 76a the incoming compressed air, after passage throughthe exchanger, is drawn on for the cooling means. A small portion of thecompressed incoming gas after passage through heat interchange means 74ais diverted from line 75a through line'89a, control valve 90a, line 91a,check valve 92a, backwardly through filter 93a to line 79a. Theembodiment illustrated in FIG. 2 operates otherwise in the same manneras that of FIG. 1.

It is to be understood that my invention is not to be limited to any ofthe embodiments described herein for illustrative purposes but only inand by the following claims.

I claim:

1. Apparatus for separating a gaseous mixture at low temperaturecomprising: a compressor means for compressing a gaseous feed mixture; aheat exchanger means for cooling said feed mixture having'first, secnd,and third passages disposed in heat exchange relationship; separatormeans for separating the compressed and cooled gaseous feed mixture intocompo- I nents; a first conduit means for conducting a compressedgaseous feed mixture from said compressor means; diverter valve meansfor alternately connecting said first conduit means to said first andsecond passages at one end thereof for passing said compressed gaseousfeed mixture alternately through said first and second passages; asecond conduit means for conducting gaseous feed mixture from said firstand second passages to said separator means; a third conduit means forreturning from said separator means at least one component of saidgaseous feed mixture; a fourth conduit means for connecting the otherend of said first passage with said second conduit means; a fifthconduit means for connecting the other end of said second passage withsaid second conduit means; first and second unidirectional flow valvemeans disposed'respectively in said fourth and fifth conduit meansarranged for unidirectional flow in said fourth and fifth conduit meanstoward said second conduit means; first and second particulate filtersdisposed respectively in said fourth and fifth conduit means on theupstream sides of said first and second unidirectional flow valve means;a sixth conduit means for communicating said third conduit means withsaid other end of said first passage; a seventh conduit means forcommunicating said third conduit means with said other end of saidsecond passage; third and fourth unidirectional flow valve meansdisposed respectively in said sixth and seventh conduit means arrangedfor unidirectional flow in said sixth and seventh conduit means fromsaid third conduit means whereby actuation of said diverter valve meansalternates the streams and reverses the direction of flow in said firstand second filters; an eight conduit means; ninth conduit meanscommunicating said eighth conduit means with said fourth conduit meansat a point intermediate said first unidirectional flow valve means andsaid first filter; a tenth conduit means communicating said eighthconduit means with said fifth conduit means intermediate said secondunidirectional flow valve means and said second filter; and fifth andsixth unidirectionalflow valve means respectively disposed in said ninthand tenth conduit means arranged for flow from said eighth conduitmeans, said eighth conduit means further communicating with said thirdconduit means.

2. The apparatus as defined by claim 1 wherein said eighth conduit meansis connected to an outlet end of said third passage.

3. The apparatus as defined by claim 2 including a first throttlingmeans in said eighth conduit means for controlling the flow of fluidwithin said eighth conduit means.

4. The apparatus as defined by claim 3 including an eleventh conduitmeans communicating an inlet end of said third passage with said thirdconduit means.

-5. The apparatus as defined by claim 4 including a twelfth conduitmeans communicating the outlet end of said third passage with said thirdconduit means.

6. The apparatus as defined by claim 4 including a twelfth conduit meanscommunicating that'portion of said eighth conduit means upstream of saidfirst throttling means with said third conduit means downstream of itsunion with said eleventh conduit means; a second throttling means insaid twelfth conduit means for controlling the flow of fluid withinsaid. eighth conduit means; and a third throttling means in said thirdconduit means intermediate the connection of said eleventh and twelfthconduit means with said third conduit means.

4 t it 4

2. The apparatus as defined by claim 1 wherein said eighth conduit meansis connected to an outlet end of said third passage.
 3. The apparatus asdefined by claim 2 including a first throttling means in said eighthconduit means for controlling the flow of fluid within said eighthconduit means.
 4. The apparatus as defined by claim 3 including aneleventh conduit means communicating an inlet end of said third passagewith said third conduit means.
 5. The apparatus as defined by claim 4including a twelfth conduit means communicating the outlet end of saidthird passage with said third conduit means.
 6. The apparatus as definedby claim 4 including a twelfth conduit means communicating that portionof said eighth conduit means upstream of said first throttling meanswith said third conduit means downstream of its union with said eleventhconduit means; a second throttling means in said twelfth conduit meansfor controlling the flow of fluid within said eighth conduit means; anda third throttling means in said third conduit means intermediate theconnection of said eleventh and twelfth conduit means with said thirdconduit means.